This invention relates to a slurry for coating anodes for use in cells for the electrowinning of metals from their oxides dissolved in molten salts, and to methods for their fabrication and reconditioning, as well as aluminium electrowinning cells containing coated anodes and their use to produce aluminium.
The production of metals by the electrolysis of their oxides is usually carried out in very chemically aggressive environments. Therefore, the materials used for the manufacture of components of production cells must be resistant to attack by the environment of such cell. Anodes of cells for the production of metals by the electrolysis of their oxides dissolved in molten salts need to be resistant to attack by the electrolyte and by the oxygen which is anodically produced during electrolysis.
Unfortunately, for the dissolution of the raw material a highly aggressive electrolyte, such as a fluoride-based electrolyte is required.
The surface of the anode must be electrochemically active, substantially insoluble in the electrolyte and resistant to attacks by the nascent monoatomic oxygen and by the subsequently formed molecular oxygen gas which are anodically produced. Since monoatomic oxygen is far more aggressive than biatomic molecular gaseous oxygen, the constituents of the active surface of the anode should contain electro-catalytic materials for the reaction which forms molecular oxygen from the monoatomic oxygen to reduce monatomic oxygen attack.
The materials having the greatest resistance to oxidation are metal oxides which are all to some extent soluble in cryolite. Oxides are also poorly electrically conductive, therefore, to avoid substantial ohmic losses and high cell voltages, the use of oxides should be minimal in the manufacture of anodes. Whenever possible, a good conductive material should be utilised for the anode core, whereas the surface of the anode is preferably made of an oxide having a high electrocatalytic activity.
In the field of aluminium production, it has been described in U.S. Pat. Nos. 5,069,771, 4,960,494 and 4,956,068 (all Nyguen/Lazouni/Doan), and U.S. Pat. No. 5,510,008 (Sekhar/Liu/Duruz) that a metal core could be protected by barrier layers and/or by oxidised metals but these results have not as yet been commercially and industrially applied.
An object of the invention is to provide a method for coating an anode for metal electrowinning cells, in particular aluminium electrowinning cells, which substantially reduces the consumption of the active anode surface that is attacked by nascent monoatomic oxygen by enhancing the reaction of nascent oxygen to gaseous molecular gaseous oxygen.
Another object of the invention is to provide a slurry for coating anodes for metal electrowinning cells, in particular aluminium electrowinning cells, which provides a coating with high electrolytic activity, a long life and which can be re-coated onto the anode as soon as such activity decreases or when the coating is worn out.
A major object of the invention is to provide an anode for metal electrowinning cells, in particular aluminium electrowinning cells, which has no carbon so as to eliminate carbon-generated pollution and reduce the cell voltage and the high cost of cell operation.
The present invention concerns a method of applying a slurry onto a conductive, heat resistant anode substrate to form an oxide coating on those parts of the substrate which are exposed to oxidising or corrosive cell environments.
The invention in particular relates to a method of coating an electronically conductive and heat resistant substrate of a non-carbon metal-based anode of a cell for the electrowinning of metals from their oxides dissolved in molten salt, to protect and make the surface of the anode substrate active for the oxidation of the oxygen ions present in the electrolyte. The method comprises applying onto the substrate a slurry comprising at least one oxide or a precursor thereof as a non-dispersed but suspended particulate in a colloidal and/or inorganic polymeric carrier, the slurry is then solidified and made adherent to the substrate upon heat treatment to form an adherent, protective, predominantly oxide-containing coating.
An oxide may be present in the oxide-containing coating as such, or in a multi-compound mixed oxide and/or in a solid solution of oxides. The oxide may be in the form of a simple, double and/or multiple oxide, and/or in the form of a stoichiometric or non-stoichiometric oxide.
A typical application for this method is the coating of anodes for the electrowinning of aluminium by the electrolysis of alumina dissolved in a molten fluoride-containing electrolyte, such as a cryolite-based electrolyte or cryolite.
The colloidal and/or inorganic polymeric carrier may be selected from alumina, ceria, lithia, magnesia, silica, thoria, yttria, zirconia, tin oxide, zinc oxide and mixtures thereof.
Advantageously, the colloidal and/or inorganic polymeric carrier forms upon heat treatment the same chemical compound as the non-dispersed particulate.
The oxides which may be used as a non-dispersed particulate and/or as a carrier may be in the form of spinels and/or perovskites or precursors thereof. Spinels may be doped, non-stoichiometric and/or partially substituted spinels, the doped spinels comprising dopants selected from the group consisting of Ti4+, Zr4+, Sn4+, Fe4+, Hf4+, Mn4+, Fe3+, Ni3+, Co3+, Al3+, Cr3+, Fe2+, Ni2+, Co2+, Mg2+, Mn2+, Cu2+, Zn2+ and Li+.
The spinels may comprise a ferrite which can be selected from cobalt, copper, chromium, manganese, nickel and zinc ferrite, and mixtures and precursors thereof. The ferrites may also be doped with at least one oxide selected from chromium, titanium, tin, zinc and zirconium. Nickel-ferrite is a preferred compound for an electrochemically active coating for its high chemical resistance and may be present as such or partially substituted with Fe2+.
Alternatively, the spinels may also comprise a chromite which can be selected from iron, cobalt, copper, manganese, beryllium, calcium, strontium, barium, yttrium, magnesium, nickel and zinc chromite, and mixtures and precursors thereof.
The slurry advantageously comprises one or more electrocatalysts or a precursor thereof, however such a constituent is not always necessary. When an electrocatalyst is used, it may be advantageously selected from iridium, palladium, platinum, rhodium, ruthenium, silicon, tin, zinc, Mischmetal oxides and metals of the Lanthanide series, and mixtures and compounds thereof.
For the formation of the coating onto the substrate, the oxide constituents of the slurry may react among themselves. Alternatively the constituents of the slurry may react with constituents of the electronically conductive and heat resistant substrate. However, a reaction is not always necessary for the formation of the coating from the slurry.
The slurry may be applied onto the substrate by conventional techniques such as brushing, spraying dipping, electrodeposition or by using rollers.
The substrate can be chosen among metals, alloys, intermetallics, cermets, and conductive ceramics. It may for instance comprise at least one of chromium, cobalt, hafnium, iron, molybdenum, nickel, copper, niobium, platinum, silicon, tantalum, titanium, tungsten, vanadium, yttrium and zirconium, and their combinations and compounds.
The substrates may advantageously have a self-healing effect, i.e. when exposed to electrolyte the substrate passivates under the effect of the electrical current and becomes substantially inert to the electrolyte.
The adherence of the coating on the substrate may be enhanced by applying onto the substrate a pre-coat before applying the slurry. Several methods are known to obtain an oxide pre-coat on a metal substrate, e.g. heating in air for prolonged periods at high temperatures ( greater than 1000xc2x0 C.).
However, a preferred pre-coat can be formed by applying a metal oxide in a colloidal or polymeric solution onto a clean metal substrate, drying and heat-treating the pre-coat at 500xc2x0 C. Oxides for the pre-coat may be selected from SiO2, Al2O3, ThO2, ZrO2, SnO2, TiO2 and CeO2. Preferably the colloid/polymer contains cerium oxide having a crystallite size of about 5 to 10 nanometer and a NO3xe2x88x92/CeO2 mole ratio of approximately 0.25, which can be prepared by following the teachings of U.S. Pat. No. 4,356,106 (Woodhead/Raw).
The pre-coat can be applied from a colloidal dispersion having a concentration between 25 and 250 g/l. Conventional techniques such as dipping, brushing or spraying can be used prior to drying and/or heat-treating the pre-coat.
The invention also relates to an anode coating slurry for coating an electronically conductive and heat resistant substrate of a non-carbon metal-based anode for the electrowinning of metals from their oxides dissolved in molten salts, to form an adherent, protective, predominantly oxide-containing coating after heat treatment and to make the surface of the anode active for the oxidation of the oxygen ions present in the electrolyte. The slurry comprises at least one oxide or oxide precursor as a non-dispersed but suspended or suspendable particulate in a colloidal and/or inorganic polymeric carrier.
This method may also be applied for reconditioning a non-carbon metal-based anode with a slurry as described hereabove, the active coating of which anode has become non-active or worn out. The method comprises clearing and restoring the surface of the conductive substrate before applying the slurry onto the substrate as described hereabove.
Another aspect of the invention is an anode of a cell for the electrowinning of a metal, in particular of an aluminium electrowinning cell, comprising an electronically conductive substrate and a protective electrochemically active coating obtained from a slurry as described hereabove.
A further aspect of the invention is a cell for the production of a metal by the electrolysis of its oxide dissolved in a molten salt, in particular for the electrowinning of aluminium or a lanthanide such as neodymium, having at least one anode comprising an electronically conductive substrate and a protective electrochemically active coating obtained from a slurry as described hereabove.
An aluminium electrowinning cell may advantageously comprise at least one aluminium-wettable cathode. The cell may be in a drained configuration by having at least one drained cathode on which aluminium is produced and from which aluminium continuously drains. The cell may be of monopolar, multi-monopolar or bipolar configuration. A bipolar cell may comprise the anodes as described above as a terminal anode or as the anode part of a bipolar electrode.
Preferably, the aluminium electrowinning cell comprises means to improve the circulation of the electrolyte between the anodes and facing cathodes and/or means to facilitate dissolution of alumina in the electrolyte. Such means can for instance be provided by the geometry of the cell as described in co-pending application PCT/IB98/00161 (de Nora/Duruz) or by periodically moving the anodes as described in co-pending application PCT/IB98/00162 (Duruz/Bellò).
The aluminium electrowinning cell may be operated with the electrolyte at conventional temperatures, such as 950 to 970xc2x0 C., or at reduced temperatures as low as 750xc2x0 C.
Yet another aspect of the invention is a method of electrowinning aluminium in a cell comprising at least one coated non-carbon metal-based anode as described hereabove, the method comprising dissolving alumina in the electrolyte and then electrolysing the dissolved alumina to produce aluminium.
The slurry as described hereabove can be used for coating a non-carbon metal-based anode for the production of aluminium in a cell for the electrowinning of aluminium by the electrolysis of alumina dissolved in a fluoride-containing electrolyte, on which anode oxygen ions in the electrolyte are oxidised and released as biatomic molecular gaseous oxygen by the electrochemically active anode slurry-obtained coating.